1. Field of the Invention
This invention relates to astigmatism corrected gratings for plane grating-spherical mirror spectrometric instruments, and more specifically to astigmatism corrected plane gratings for use in the Ebert, Ebert-Fastie and Czerny-Turner class of spectrometric instruments.
2. Brief Description of the Prior Art
Many different types of spectrometric instruments have been used for well over a century for many different purposes such as, for example, spectrometers for determining the wavelengths of light passed through materials to gain knowledge of the characteristics or chemical compositions of such materials or for determining the characteristics of the light source. Most spectrometers use diffraction gratings to disperse the light by its respective wavelength components. The diffraction gratings are usually of the reflective type and have a flat reflective surface with a series of diffractive elements, such as parallel lines or rulings inscribed thereon and spaced closely enough together to cause diffraction of the wavelengths of interest by means of constructive and destructive interference. Reflective diffraction gratings have proven very useful and desirable in spectroscopy because of their lack of chromatic aberration, their ability to proportionally disperse light by wavelength, even at large angles, and their ability to disperse very wide spectral ranges.
Diffractive grating spectrometers can also be used as monochromators, which can be thought of as precise optical filters. Essentially, the layout of the monochromator is very similar to that of a conventional spectrometer, except that an exit pupil or slit is sized and positioned in a manner that blocks out all but a specific wavelength of the diffracted light. In this manner, monochromators are useful as sources of monochromatic light to study the effects that one wavelength of light has on other elements. For example, monochromatic light from a monochromator may be passed through a gaseous sample to excite the various atoms therein. Light emitted from the excited atoms may then be analyzed by passing it through a spectrometer. Such emission spectroscopy often requires a fairly energetic monochromatic light source to properly excite a sample so that it emits sufficient light. Because of this high energy requirement, monochromators are sought to have as much light output, or throughput, as possible.
The particular class of diffractive spectrometric instruments that are related to the present invention are generically referred to as plane grating-spherical mirror instruments. Because of their ruggedness and simplicity, these plane grating-spherical mirror instruments have proven to be the workhorses of laboratory, field, and remote sensing applications. Instruments falling into this particular class include configurations or mountings known as Ebert, Ebert-Fastie, and Czerny-Turner. Generally speaking, these instruments comprise an entrance slit, one or more concave spherical mirrors, a plane diffraction grating, and an exit slit or photographic plate in the plane of the entrance slit. The primary advantages associated with these instruments is that they combine high spectral resolution and acceptable throughput with relatively simple and rugged optic designs that are easy to assemble and adjust. Therefore, these designs have been the preferred choice for many applications, including monochromators and spectrometers for research in atomic and molecular spectroscopy and for use on spacecraft.
The Ebert spectrometer was first described by Hermann Ebert in 1889. Essentially, Ebert's spectrometer consisted of an entrance slit, a single concave spherical mirror, a plane diffraction grating, and a small photographic plate in the plane of the entrance slit. The single concave mirror was used to both collimate and focus the light. In 1930, Czerny and Turner used a modification of the Ebert system in which the collimating and focusing were performed by two separate mirrors instead of by the two halves of the single, large mirror originally used by Ebert. This so-called Czerny-Turner system eliminated coma, since the coma distortion of the wave front arriving at the off-axis grating was canceled by the symmetrically off-axis focusing mirror. Fastie replaced the photographic plate by an exit slit to form a monochromator. Hence, these spectrometric instruments may be referred to as Ebert, Czerny-Turner, or Eben-Fastie, depending on their particular mountings or configurations.
Unfortunately, because the mirror or mirrors of these spectrometric systems are always used off-axis, they introduce a significant amount of astigmatism into the diffracted image. That is, the tangential and sagittal foci, which represent components of light in orthogonal planes, do not occur at the same point in space. This astigmatism causes a substantial loss in the intensity of the diffracted beam as the resolved light in each wavelength or band is stretched out in a protracted line image instead of being concentrated at a point. This loss of intensity and stretching out of the wavelength band results in a total loss of any information relating the diffracted image to specific height positions along the entrance slit. Thus, imaging spectroscopy, where simultaneous spectroscopy of two sources or a complex image falling on the entrance slit, is impossible with uncorrected diffractive spectrometers of this class. The loss of intensity also reduces the available light or throughput in monochromator applications, thereby further limiting usefulness. Moreover, the increased resolving power of new gratings requires the use of narrower lines which further accentuates the intensity loss problem.
Fastie worked to minimize the effect of astigmatism and compensate for spectral line curvature for all wavelengths by using curved slits that formed arcs of the same circle. Fastie also used an "over-and-under" variation of the original Ebert mounting in which the photographic plate was mounted above, and the slit was mounted below the level of the grating. Unfortunately, Fastie's efforts in reducing the effects of astigmatism have been mainly directed at increasing net throughput. The long curved slits of his design also allow isochromatic astigmatic images to pass through without significant loss of spectral resolution. True astigmatism reduction has traditionally taken a back seat to studies aimed at reducing aberrations that are more directly related to spectral resolution, such as developing new methods of precisely optimizing the mirror radii, optic positions, and aperture stop positions.
While these plane grating-spherical mirror spectrometric instruments do have the disadvantage of introducing substantial astigmatism into the diffracted image, as described above, they have such excellent resolution, and are so simple and rugged that the disadvantages of the astigmatism have generally been tolerated for most applications. However, if the astigmatism could be eliminated, detection efficiency, throughput, and signal-to-noise ratio could be improved, because both the sagittal and tangential foci would be brought together to converge the diffracted image to a focused, high luminosity point instead of the protracted line. Also, comparative spectroscopy, where two sources are projected through two halves of the same entrance slit, would also be enhanced. Stigmatic imaging could also allow for two-dimensional spectroscopy, that is, where spectral and slit position information are gathered simultaneously. Thus, a person or electronic detector equipment could identify the light spectrum or wavelength bands present or absent at each point along an elongated entrance slit, instead of having to use a point entrance or source. Such an elongated entrance slit could, for example, be wiped or moved across an image of the sun or across a candle flame or other non-point light source, while taking continuous or intermittent real time measurements of wavelengths at selected points along the entrance slit or even continuously along the entrance slit to obtain a two-dimensional reading of all the wavelengths present at all points on the sun or candle flame image. Further, continuing development of miniature multi-element detection systems have greatly enhanced the desirability of systems having "spatial imaging" abilities as described above. The limitations caused by the astigmatism threaten to make this simple but efficient class of spectrometric instruments obsolete.
Prior to this invention, there have only been two options available to reduce the amount of astigmatism associated with this class of instruments. The first of these previously known methods was discovered by Fastie and, as described above, relies on curved slits that form arcs of the same circle. However, this method only minimizes the effects of astigmatism on throughput, and does not really get to the root of the problem of converging the sagittal and tangential foci.
In the second alternative, the spherical mirrors in the Czerny-Turner instruments have been reshaped or distorted in an attempt to achieve stigmatic or near stigmatic imaging. By varying the shapes of the mirrors to something other than an exact spherical surface, for example by closing the radius of the mirror in one dimension, it has been possible to reduce the astigmatism in the instrument. Such aspherical surfaces, usually ellipsoidal or toric, have been used with limited success. However, it is relatively difficult and expensive to properly "distort" i.e., reshape the spherical mirrors, and such mirror reshaping can only be practically done in a two-mirror Czerny-Turner system. That is, it is difficult, if not impossible to correctly distort the single mirror of the Ebert mounting to reduce astigmatism, because the distortions would have to be carried out at two locations on the mirror. Furthermore, it is difficult to realign the two distorted mirrors of the modified Czerny-Turner system to optimize stigmatic imaging, which difficult re-alignment removes some of the advantages of ruggedness and simplicity associated with this class of instruments. Therefore, this mirror reshaping technique has only enjoyed limited success in improving the imaging qualities of these instruments.
Until this invention, spectroscopists had generally concluded that astigmatism, since it is present in all off-axis uses of spherical reflecting surfaces, is inescapable with plane grating-spherical mirror spectrometric instruments. Spectroscopists have generally accepted the astigmatism as the price to be paid for the high resolution, high throughput, simplicity and ruggedness of these instruments.